The embodiments disclosed herein relate generally to the field of heat cycle system for recovering waste heat, and more particularly, to a compound closed-loop heat cycle system having a Brayton top cycle and a Rankine bottom cycle for recovering waste heat, and method thereof.
Enormous amounts of waste heat are generated by a wide variety of industrial and commercial processes and operations. Example sources of waste heat include heat from space heating assemblies, steam boilers, engines, and cooling systems. The term “waste heat” encompasses any supply of residual heat given off by a primary processes that is not conventionally exploited as a source of energy.
Some power generation systems provide better reliability and off-grid operation with alternative fuels such as biogas or landfill gas, with examples being gas turbines and combustion engines such as microturbines and reciprocating engines. Combustion engines may be used to generate electricity using fuels such as gasoline, natural gas, biogas, plant oil, and diesel fuel. However, atmospheric pollutants such as nitrogen oxides and particulates may be emitted.
One method to generate electricity from the waste heat of a combustion engine without increasing emissions is to apply a bottoming steam Rankine cycle. A Rankine cycle typically includes a turbo generator, an evaporator/boiler, a condenser, and a liquid pump. However, water-based steam Rankine cycles are not attractive in the aforementioned low temperature waste heat region due to high cost and low efficiency. The performance of an organic Rankine cycle (ORC) is limited by constraints of the working fluid circulated within the ORC. Steam used as a working fluid may be optimal only for a specific range of cycle temperatures and pressures. This conventional steam Rankine bottoming cycle requires condensation at relatively low pressure, implying large low-pressure turbine and condenser volumes. Hence installation of conventional bottoming steam Rankine cycle system is disproportionately bulky, and complex considering the relatively small yield derived from low-temperature waste heat. The low pressure of the steam condensation introduces other complexities, such as the need for special de-aeration units to remove atmospheric air that leaks into the sub-atmospheric pressure vessels from the outside.
It would be desirable to have a simple system and method that effectively recovers waste heat and that is not limited by constraints of a steam working fluid circulated within a Rankine cycle system.